This invention relates to new and useful acidcurable resin compositions which comprise a mixture of furan resin and a minor amount of an epoxy resin. These curable resin compositions are useful particularly in preparing formed, shaped, filled bodies which comprise the acid-curable resin composition and inorganic solid particulates. Particularly useful filled bodies of this type include foundry cores and molds.
Furan resins and furan polymers are terms often used interchangeably referring to thermosetting resins which are obtained primarily by condensation polymerization of furfuryl alcohol or furfuryl aldehyde in the presence of strong acids. The term also generally includes resins made by condensing phenol with furfuryl alcohol or furfuryl polymers or furfuryl-ketone polymers. Examples of furan resins include furfural-phenol resins, furfuryl alcohol resins, furfuryl-acetone resins, furfuryl alcohol-formaldehyde resins, etc.
Generally, the curing of furan resins can be accomplished by the addition of either mineral or organic acids with the rate of cure primarily dependent upon temperature and the activity and concentration of the acid catalyst. Strong catalysts such as sulfuric acid or para-toluene sulfonic acid can effect rapidcures at about room temperature with an exothermic reaction to a peak temperature of about 90.degree. to 100.degree. C.
Furan resins have been used as binders in foundry applications, grinding wheels, fiberglass reinforced plastic laminated equipment, and for coated abrasives and polymer concrete. In many of these applications, it is desirable to be able to work with the binder system in the uncured, fluid state for an extended period of time prior to reaching the cured or unworkable state.
Furan-based binders are desirable in foundry applications, and particularly in core production. After pouring of the molten metal over the core or mold, sand shapes prepared utilizing furan-base binders rapidly deteriorate and result in an easy "shake-out".
It also is desirable to provide curable foundry binder systems which are exothermic and which may be triggered by relatively low levels of heat input. The foundry industry continues to seek binder systems and methods for rapidly producing foundry cores and molds which are of high quality and which exhibit high tensile strengths, good chemical resistance and increased resistance to stress cracking. The utility of furan resins as binders and foundry applications is illustrated in U.S. Pat. Nos. 4,439,348; 4,451,577; 3,879,339; etc.
Modified furan binder systems comprising mixtures of furan resins with other resins also have been described. U.S. Pat. No. 4,176,114 describes the formation of sand cores or molds from sand and condensation-type resins comprising a high viscosity of polyfurfuryl alcohol or mixtures of polyfurfuryl alcohol with a phenol-formaldehyde, urea-formaldehyde, or phenol-urea-formaldehyde precondensate resin, or a condensate of polyfurfuryl alcohol with a phenolformaldehyde, urea-formaldehyde, or phenol-urea-formaldehyde precondensate resin. Curing of the resin system is effected by acid catalysts such as sulfuric acid formed in situ.
Modification of the properties of furan resins as binders in foundry applications also is described in U.S. Pat. No. 4,215,206. Furan resins are modified by incorporating an aromatic polyester polyol derived from polycarbomethoxy-substituted diphenyl, polyphenyl and benzyl esters of the toluate family. This combination is reported to provide the furan binders with a significant increase in core strength.
The so-called "epoxy" resins have been utilized as binders and adhesives in a number of applications. Epoxy resins are characterized by the presence of an epoxide group, i.e., ##STR1## Epoxy resins are available commercially from a wide variety of sources, and the commercially available products, depending upoon the chemical make-up of the epoxy resin, are characterized by a variety of properties.
Although furan resins have been utilized extensively as binders in foundry applications, it generally has been observed that the structures prepared utilizing such furan binders are typically very hard and brittle with poor elongation, and the cured structures are subject to stress cracking. Furan binders and various aggregates have been used in construction of chemical resistant floors, tanks, curbings and related structures, but persistant stress cracking problems and difficulties in application have prevented their widespread use.